1. Field of the Invention
This invention relates generally to a system and method for controlling the output power or current from a fuel cell stack at power up-transients and, more particularly, to a system and method for limiting stack output power or current during power up-transients so as to allow an anode exhaust gas recirculation pump to maintain the ratio of recirculated anode exhaust gas to fresh hydrogen substantially constant.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membrane in a fuel cell needs to have a certain relative humidity so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The flow of the reactant gas through the flow channels has a drying effect on the membrane, most noticeably at an inlet of the flow channels. Also, the accumulation of water droplets within the flow channels from the membrane relative humidity and water by-product could prevent reactant gas from flowing therethrough, and cause the cell to fail, thus affecting the stack stability. The accumulation of water in the reactant gas flow channels is particularly troublesome at low stack output loads.
It is desirable that the distribution of hydrogen within the anode flow channels in a fuel cell stack be substantially constant for proper fuel cell stack operation. Some fuel cell systems input more hydrogen into the fuel cell stack than is necessary for a certain output load of the stack so that the anode gas is more evenly distributed. However, because of this requirement, the amount of hydrogen in the anode exhaust gas is significant, and would lead to low system efficiency if that hydrogen were discarded. Therefore, it is known in the art to recirculate the anode exhaust gas using an anode recirculation pump back to the anode input to reuse the discarded hydrogen.
When a request for high stack power is given, referred to herein as a power up-transient, a command is given to the compressor to provide a desired amount of cathode air to the stack cathode and a command is given to the anode recirculation pump and fresh hydrogen injectors to provide a desired amount of hydrogen to the stack anode to satisfy the power request. Typically, the compressor is able to provide the cathode air very quickly to meet the cathode input requirements and the hydrogen injectors are able to provide the fresh hydrogen very quickly. However, the time it takes the anode recirculation pump to spin up is slower, which for some period time after the power up-transient is requested, prevents the desired amount of recirculated anode exhaust gas from being sent to the anode inlet side of the stack. This can temporarily reduce the performance of the stack because of local drying of the anode inlet region. Further, the ratio of fresh hydrogen to recirculated anode exhaust gas changes where more fresh hydrogen is provided relative to the recirculated anode exhaust gas. Because the fresh hydrogen is dry and the recirculated anode exhaust gas is humidified, the humidity level within the fuel cells goes down especially in the anode inlet region, which causes the membranes in the fuel cells to at least partially dry out, which can induce mechanical stress that may lead to pinholes or other perforations being formed in the fuel cell membranes. Furthermore, in this particular situation, i.e., up-transient, the desired anode gas volume flow may not be met. This can lead to local hydrogen starvation due to lower volumetric flow in the anode outlet region that can negatively impact water management. This in turn can lead to damage of the cathode catalyst support which heavily impacts fuel cell performance and durability.